Ethylene-propylene copolymers (EPM) and ethylene-propylene-diene terpolymers (EPDM) are extensively utilized in a variety of applications. A particular application wherein EPM and EPDM are preferred because of their excellent physical properties, weathering and heat aging resistance is in rubber sheeting, such as roofing, agricultural pond liners and water distribution membranes. EPM and EPDM sheeting materials usually are prepared by compounding the EPM, EPDM or mixtures thereof with the appropriate fillers, plasticizers, antidegradants, etc. in a suitable internal mixer, and calendering the resulting compound into the desired sheet rubber. In order to achieve good calendering at economically suitable productions rates the EPM/EPDM formulations should possess, among other qualities, a fairly high green strength. Conventional methods of increasing the green strength of EPM/EPDM formulations include utilizing EPM and/or EPDM polymers having a relatively high ethylene content, using EPM and/or EPDM polymers having relatively high molecular weights, and utilizing EPM and/or EPDM polymers having a broad molecular weight distribution. While these methods generally provide EPM and/or EPDM formulations exhibiting some improvement in green strength, they place limits on the types of EPDM and/or EPM polymers which can be used, and consequently place relatively restrictive limits on the desired properties which the sheet material can possess. Moreover, the foregoing methods of improving the green strength of EPM and/or EPDM formulations generally have accompanying disadvantages such as poor low temperature processability, mixing and pigment dispersibility problems, slower cure rates, poor cured physical properties, or combinations thereof.
U.S. Pat. No. 5,247,009 to Kitahara discloses a rubber composition which includes a rubbery polymer which can for example be EPDM and a modified liquid polymer which is obtained by reacting a liquid polymer having at least one carbon-carbon double bond with an organic compound having a group of the formula --CH.dbd.N-- and with an organic acid halide, in the presence of a Friedel-Crafts catalyst. The patent discloses that the composition exhibits better processability on the calender rolls than the comparative composition containing the rubber alone, and has improved green strength. The Kitahara patent shows modest improvements of green strength for natural rubber, polyisoprene and polybutadiene. However, while Kitahara shows improved strength properties for vulcanized EPDM rubber which has been formulated with the modified liquid polymers, data showing how green strength is affected when the modified liquid polymers are added to EPDM are conspicuously absent. Moreover, the composition has the disadvantage of requiring a modified liquid polymer which must be prepared and separated from a reaction mixture, thereby significantly increasing the steps and expense of preparing a formulation believed to exhibit modest, if any, improvement in green strength.
Various attempts have been made to increase the strength of cured polymeric formulations by adding reinforcing mineral fillers thereto. Such materials have also been added to rubber formulations to increase hardness, green stock viscosity and specific gravity, and to reduce the modulus of the cured compounds. However, it is believed that these mineral fillers have not been used nor are they known for improving the green strength and processability, particularly during Banbury mixing and the calendering of rubber formulations prior to vulcanization.
U.S. Pat. No. 2,422,153 to Van Nimwegan et al. discloses the use of various reactive materials for improving the strength of cured rubber articles. Examples of such reactive materials include boric oxide, vanadium oxide, boraluminates, borates, phosphates, sulfates, nitrates and halides. The Van Nimwegan et al. patent, however, does not disclose materials which are expected to improve green strength to facilitate increased processing rates.
U.S. Pat. No. 3,976,624 to Inouye et al. discloses the use of metal borates, including calcium borate, to improve adhesion between a rubber composition and a metal surface which has been coated with the rubber composition. The Inouye et al. patent relates primarily to SBR and other rubber compositions which are used to form tire components that are bonded to metal cords or the like, and does not suggest the use of borates in EPM, EPDM, or in any rubber formulation which is not intended or required to be bonded to metal substrates.
Thus, while minor improvements in green strength can be achieved utilizing specific EPM or EPDM polymers which generally adversely affect other desirable properties, and while improvements in the strength of cured rubber compositions can be achieved by utilizing various reinforcing fillers or reactive materials, there still exists a need for an effective, inexpensive green strength promoter for EPDM and/or EPM compositions, especially if the EPDM and/or EPM is an amorphous (non-crystalline) polymer.